Exhaust gas stack

ABSTRACT

An exhaust gas stack includes an elongate, tubular stack member having inlet and outlet ends and a central longitudinal axis. An end section is mounted on the outlet end and has open first and second ends, the first end connected to the outlet end of the stack member and a frustoconical passage extending between these ends. An inner duct section has a central axis co-axial with the axis of the stack member and is mounted centrally in the end section. An annular gas passage is formed between the duct section and the end section. An air plug is movable between an open position at which exhaust gases can flow through the inner duct section and another position at which the plug restricts or prevents passage of exhaust gases through this duct section. An actuator is connected to the plug in order to move it between an open and other positions.

BACKGROUND OF THE INVENTION

The present disclosure is directed to exhaust gas stacks and to apparatus and devices for controlling the velocity of exhaust gases exiting from exhaust stacks.

A variety of exhaust gas stacks and exhaust ducts are known in industry for the purpose of exhausting gases from sources such as furnaces. Vertical exhaust stacks can vary in height and in some cases can be of substantial height. Existing conventional exhaust gas stacks are generally constant geometry (fixed discharge area) devices. In many cases, such stacks are sized so as to maintain approximately 4,000 fpm velocity at the stack discharge at the maximum flow rate for the particular exhaust application in order to diffuse the exhaust gases into the atmosphere as required by local building codes.

With a conventional stack design, the velocity of the gases at the stack discharge will drop when the airflow volume changes to lower than maximum since the discharge area of the stack remains constant. To explain further, in the case of an exhaust stack for the removal of air from a number of laboratories, the airflow rate for the exhaust will be reduced when fewer laboratories are being used since fewer laboratories will generate a lower amount of exhaust air flow. One known method of overcoming this difficulty is the introduction of outside air into the exhaust air system in order to bring the total flow rate to the maximum design value for the system, thereby maintaining the required discharge velocity at the top end of the stack. This is accomplished by using outside air dampers with the air intake plenum. As the exhaust air quantity drops, these dampers are opened to introduce outside air into the fan inlet chamber so that the fan delivers constant airflow at all times. However, the operation of this exhaust system is not energy efficient.

Another concern with a standard exhaust stack design is the amount of noise that eminates from the exhaust stack. This is becoming a more significant problem for industry and for laboratories because the allowable noise levels that can be radiated to the environment is becoming more restricted by local by-laws and other legislation.

Various systems for throttling or controlling the flow of exhaust gases from a chimney or stack are provided in the patent literature. For example, U.S. Pat. No. 3,978,776 to R. Kausche describes an exhaust gas chimney which, at its top end, is fitted with a first upwardly converging funnel having an outlet for the flow of exhaust gases and a second upwardly converging funnel co-axial with the first funnel but of smaller size. There is a mechanism for moving the funnels relative to each other for throttling the flow of exhaust gases between the funnels. In a closed position, the funnels adjoin each other to form an outlet of a size smaller than the outlet of the first funnel.

U.S. Pat. No. 3,537,411 to J. R. Roy describes a double shelled chimney stack made with a metal load-bearing outer shell and a nonload-bearing, gas conveying metal inner shell. A hermetically sealed annular air space between the shells provides insulation. This air space comprises a plurality of axially spaced annular spaces located between the shells and the top of the stack has a frustoconical cap for increasing the velocity of the gases.

SUMMARY OF THE PRESENT DISCLOSURE

The present disclosure provides an advantageous exhaust gas stack for emitting exhaust gases into the atmosphere and an advantageous velocity control apparatus for use at the top end of an exhaust stack which is able to adjust or control the exit velocity of exhaust gases. As will be apparent from the following detailed description and appended figures that follow, the present disclosure provides, inter alia, the following advantageous advances to the field of stacks or chimneys for exhaust gases:

-   -   (a) An exhaust gas stack or a velocity control apparatus for use         with such a stack wherein the net opening area of the stack can         be varied in order to maintain a predetermined exhaust airflow         velocity at the outlet of the stack;     -   (b) An exhaust gas stack with sound attenuating features that         enable quiet operation; and     -   (c) An exhaust gas stack that provides energy efficiency as it         can permit the use of an exhaust fan powered by a smaller motor,         i.e. lower horsepower motor than the fan/motor combination         required by a conventional exhaust gas stack designed for         similar exhaust airflow rates.

According to an exemplary embodiment of the present disclosure, an exhaust gas stack comprises an elongate, tubular stack member having an inlet end, an outlet end, a central longitudinal first axis and an exhaust gas passage extending between the inlet and outlet ends. The stack member is adapted to extend vertically with the inlet end being a bottom end when the exhaust gas stack is installed for use. An end section is mounted on the outlet end and has an open first end, an open second end of small diameter than the first end, and a frustoconical passage extending between the first and second ends. The first end is connected to the outlet end of the stack member. An inner duct section, which is substantially circular in transverse cross-section, has a first duct end, an open second duct end, and a central longitudinal axis co-axial with the first axis. The duct section is mounted centrally in the passage of the end section whereby an annular gas flow passage is formed between the duct section and the end section. The duct section has a first axial portion adjacent the first duct end and a second axial portion adjacent the second duct end with the two axial portions meeting at an annular junction of reduced diameter relative to the diameter of the first axial portion at the first duct end. An air plug member is movable between an open position at which exhaust gases can flow past the plug member and through the inner duct section and another position at which the plug member restricts or prevents flow of exhaust gases through the inner duct section. An actuator is connected to the plug member and is capable of moving the plug member between the open position and the closed position.

In one embodiment of this exhaust gas stack, an annular duct wall forming the first axial portion of the inner duct section curves radially inwardly and upwardly as seen in cross-section through the central, longitudinal axis of the duct section and the second axial portion has a frustoconical shape which diverges upwardly from the annular junction.

According to another exemplary embodiment of the present disclosure, a velocity control apparatus for use at a top end of an exhaust fan includes a hollow member adapted for mounting on the top end of the exhaust stack and having an open first end, an open second end of smaller diameter than the first end, and a frustoconical passage extending between the first and second ends. The first end of this member is connected to the top end of the stack during use of the apparatus. An inner duct section is also provided and this section has a first duct end, an open second duct end, and a central longitudinal axis which is aligned with the central longitudinal axis of the exhaust stack when the apparatus is mounted on the exhaust stack. The duct section is mounted centrally in the hollow member whereby an annular gas flow passage is formed between the duct section and the hollow member. An air plug member is movable between an open position at which the exhaust gases can flow past the plug member and through the inner duct section and another position at which the plug member restricts or prevents flow of the exhaust gases through the inner duct section and thereby at least reduces the amount of exhaust gases flowing through the inner duct section during use of this apparatus. The apparatus includes an actuator connected to the plug member and adapted to move the plug member between the open and another position.

According to one version of this embodiment, the actuator includes a linear actuator unit adapted for mounting in the exhaust stack and an elongate connecting member capable of extending from the actuator unit to the plug member and being connected to the actuator unit so as to be movable lengthwise thereby in order to move the plug member between the open position and the another position.

According to yet another exemplary embodiment of the present disclosure, an exhaust gas stack includes an elongate, tubular stack member having an inlet end, an outlet end, and an exhaust gas passage extending between the inlet and outlet ends. The stack member is adapted to extend upwardly from the inlet end when the stack is installed for use. The stack member includes a tubular exterior wall, an interior wall made of perforated sheet metal having numerous small holes distributed over its surface and spaced inwardly from the exterior wall, and sound attenuating material sandwiched between the exterior wall and the interior wall. An elongate, airflow defining member is mounted in the stack member and has a central longitudinal axis which is co-axial with a central longitudinal axis of the stack member. The exhaust gas passage is an annular passage along the length of the airflow defining member. The airflow defining member has a sheet metal exterior shell perforated with numerous small holes distributed over at least a substantial portion of the exterior shell and containing sound attenuating material arranged within the exterior shell. There is also an end section mounted on the outlet end of the stack member and having an open first end, an open second end of smaller diameter than the first end, and a frustoconical passage extending between the first and second ends. The first end is connected to the outlet end of the stack member. There is also provided an exhaust gas velocity control mechanism having an inner duct section which is co-axial with and mounted in the passage of the end section. An air plug member is movable between an open position at which exhaust gases can flow through the inner duct section and another position at which the plug member restricts or prevents passage of exhaust gases through the inner duct section. A further passage is formed around the inner duct section between this duct section and the end section.

These and other aspects of the disclosed exhaust gas stacks and velocity control apparatus for use on an exhaust stack will become more readily apparent to those having ordinary skill in the art from the following detail description taken in conjunction with the accompanying-drawings.

BRIEF DESCRIPTON OF THE DRAWINGS

So that those having ordinary skill in the art to which the present disclosure pertains will more readily understanding how to make and use the subject invention, exemplary embodiments thereof will be described in detail herein below with reference to the drawings, wherein:

FIG. 1 is an elevational view along an axial cross-section of an exemplary exhaust gas stack according to the present disclosure, this view showing an air plug member in a fully closed position;

FIG. 2 is an elevational view along an axial cross-section similar to FIG. 1, this view showing the plug member in a fully open position;

FIG. 3 is a top view of the top end of the exhaust gas stack of FIG. 1;

FIG. 4 is a horizontal cross-section along the line IV-IV of FIG. 1;

FIG. 5 is a horizontal cross-section taken along the line V-V of FIG. 1;

FIG. 6 is a side view of an upper section of an exhaust gas stack which has been fitted with an induction collar mounted at the top end of the stack;

FIG. 7 is an elevational view of two exemplary exhaust gas stacks mounted one beside the other with the stacks being shown in axial cross-section and mounted on top of a housing and respective vane-axial fans; and

FIG. 8 is a schematic elevational view of the exhaust gas stack showing booster relay and control connections.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIGS. 1 and 2 illustrate one exemplary embodiment of an exhaust gas stack constructed in accordance with the present disclosure. This exhaust gas stack includes an elongate, tubular stack member 12 having an inlet end 14, an outlet end 16, a central longitudinal first axis indicated at A and exhaust gas passage 18 extending between the inlet and outlet ends. The stack member is adapted to extend vertically with the inlet end being a bottom end when the stack is installed for use. The illustrated embodiment of the stack member has an exterior wall 20 which can be made of imperforate sheet metal such as sheet steel having sufficient thickness and strength to support the weight of the stack and external forces acting thereon such as wind. The illustrated stack member 12 has a square, horizontal cross-section defined by this exterior wall but it will be appreciated that the exterior horizontal cross-section can also be circular along the entire length of the stack member. Spaced inwardly from each side of the exterior wall is an interior wall 22 which can be made of perforated sheet metal having numerous small holes distributed over its surface. The interior wall can also be made of sheet steel. The interior wall can be connected to the exterior wall by means of suitable connecting brackets (not shown). The space 24 between each interior wall and the exterior wall can be filled with sound attenuating material in order to reduce the level of sound that eminates from the stack. Common forms of sound attenuating material that can be used include fiberglass batting and mineral wool. This material is protected from rainwater or other moisture in the stack by a moisture proof plastic barrier layer that extends between this material and the interior wall 22. Suitable plastic materials for this purpose are those sold under the trademarks TEDLAR™ AND MYLAR™. These materials do not interfere significantly with the sound attenuation. Horizontal bars or tubes 26 mounted in the space between the interior and exterior walls can help support the sound attenuating material in each side wall of the stack. Arranged at the top end or outlet end of the stack member is an end section 28, this end section having an open first end 30, an open second end 32 of smaller diameter than the first end, and a central axis which is co-axial with the axis A. The first end is connected to the outlet end 16 of the stack member. Preferably the end section 28 is tapered from its first end 30 to its second end as shown. More preferably the end section 28 has a frustoconical exterior. Like the stack member, the end section 28 can also be of double-walled construction. Thus, in the illustrated embodiment, the end section has an imperforate exterior wall 36 which forms a small acute angle with the vertical plane on each of the four sides of the stack and a perforated interior wall 38, the latter forming a frustoconical gas passage. Again, sound attenuating material can be stuffed or placed in the space 40 formed between the interior and exterior walls. The interior and exterior walls can be connected to each other by means of horizontally extending metal tubes 42 located at the top and bottom ends of these walls.

As part of a velocity control apparatus for the stack 10, there is provided an inner duct section indicted generally at 44. This duct section can be substantially circular in transverse cross-section and has a first duct end 46, an open second duct end 48 and a central longitudinal axis co-axial with the first axis A. This duct section 44 is mounted centrally in the passage of the end section 28 whereby an annular gas flow passage is formed between the duct section and the end section, this passage indicated at 50. The duct section 44 has a first axial portion 52 adjacent the first duct end and a second axial portion 54 adjacent the second duct end, with the two axial portions meeting at an annular junction 56 of reduced diameter relative to the diameter of the first axial portion at the first duct end 46. The first axial portion 52 of the inner duct section curves radially inwardly and upwardly as shown in FIGS. 1 and 2, that is, in cross-section through the central longitudinal axis of the duct section. A lower section of the first axial portion can extend vertically from the first duct end before the wall of the duct section curves inwardly. Also, in the embodiment of FIG. 1, the second axial portion 54 has a frustoconical shape which diverges upwardly from the annular junction 56. The annular junction can form a rounded interior surface as shown. The top end 48 of the duct section can be aligned with the top or second end 32 of the end section 28. The first or lower duct end 46 of the duct section can project a short distance into the stack member 12.

The stack is fitted with an air plug member 60 which is movable between an open position at which exhaust gases can flow past the plug member and through the inner duct section 44 and a closed position at which the plug member engages an interior surface of the inner duct section at the annular junction and thereby prevents exhaust gases from flowing through the inner duct section. The open position of the air plug member is shown in FIG. 2 while the closed position is shown in FIG. 1. An actuator 62 is connected to the plug member 60 and is capable of moving the plug member between the open position and the closed position. In the illustrated embodiment, the plug member has a hemispherical top portion 64 with a convex exterior surface directed upwardly during use of the stack. This plug member also has a substantially cylindrical skirt 66 extending from the top portion towards the inlet end of the stack member. A flat, horizontal plate 68 can extend across the bottom of the top portion 64 and provide structural rigidity to the plug member, as well as an attachment point for the actuator 62. The plug member can also be made of a sheet metal such as sheet steel. In an exemplary embodiment of the stack, the convex exterior surface of the plug member extends parallel to but is spaced apart from the annular duct wall of the duct section 44 where the duct wall curves inwardly when the plug member is in the open position. This relationship can be seen clearly in FIG. 2. Thus, in the open position, exhaust gases can flow smoothly through the annular gap 70 which is of substantially uniform width and smoothly curved.

The actuator 62 can be a linear actuator unit and can be mounted in the stack member along the longitudinal first axis A. In an exemplary embodiment, the actuator is a pneumatic actuator but it is also possible to use an electric operator or a hydraulic actuator. It will be understood that the pneumatic actuator is provided with pressurized air from a source of pressurized air by pressurized air line 170 (see FIG. 8). Although the illustrated actuator 62 is shown mounted near the inlet end 14 of the stack, it will be appreciated that it is also possible to mount the actuator further up the stack and closer to the plug member if desired. By mounting the actuator in the position shown, it can be easier to access the actuator for repair or replacement and to make the necessary connections for its support and operation. The movable rod of the actuator which extends from its top end is connected to an elongate connecting member 72 which can be a solid metal rod of one inch diameter. The member 72 is operatively connected to the actuator unit so as to be axially movable thereby in order to move the plug member between its open and closed positions. The rod member 72 can be slidably supported along its length by several bushings 73 which are rigidly mounted in a central airflow defining member 100 (described below) by means of braces 75. The exhaust stack can include an electronic controller 80 for controlling operation of the linear actuator and a flow sensor 76 located in the end section 28 for measuring the velocity of exhaust gases flowing from this end section and generating an electrical signal indicative thereof. The controller in one embodiment is a model CSC-3000 series volume reset controller sold by KMC Controls.

The flow sensor 76 can be a pitot tube such as Dwyer Model 160-8. The purpose of the sensor 76 is to provide a means to measure the velocity pressure, which can then be converted to velocity. This known type of sensor has two parts, one for measuring the total pressure and the other for measuring the static pressure. The difference between these two measurements is the velocity pressure. In one version, the flow sensor is mounted 5 inches radially in from the edge of the stack, flush with the top end 32 of the end section 28. Wires 78 and 79 shown in FIG. 8 can connect the sensor 76 to the electronic controller 80 which can be suitably housed at the bottom end of the stack. The controller can be a microprocessor or a programmable computer, which can be of standard construction. The controller 80 is programmed to operate the actuator unit 62 from electrical inputs that include the electrical signals from the sensor 76 so as to move the plug member to a controller calculated position amongst a range of positions between the open and closed positions in order to seek or maintain a desired airflow velocity for the exhaust gases flowing from the end section 28. In one version of the sensor 76, it has a total pressure connection which is connected by line 78 to the “H” terminal on the controller and has a static pressure connection which is connected by line 79 to the “L” terminal on the controller 80. The pitot tube line and head for measuring total pressure are represented by line 190 in FIG. 8. It will be seen that this line terminates outside the end section 28 of the stack. In an exemplary embodiment of the present exhaust stack and its controller, the position of the plug member 60 is adjusted so as to vary the net open area at the top end of the exhaust stack in order to maintain a selected constant velocity at all exhaust air flow rates. Thus, for example, in one exemplary embodiment, the exhaust airflow rate can vary from 9,000 fpm to 26,400 fpm while the velocity control apparatus, including the aforementioned controller, is able to maintain a velocity discharge of about 4,000 fpm at all of the operating levels in this range.

With reference now to the detail drawings of FIGS. 3 to 5, FIG. 3 illustrates features at the top end of the frustoconical end section 28. The round second duct end 48 of the inner duct section is supported on four sides by four braces 84 which can be made of ¼ inch flat bar. In one embodiment, the diameter of the end 48 is 22.5 inches while the diameter of the round top of end section 28 is at least 32.5 inches. The outer end of each brace 84 can be welded or bolted (by means of a connecting flange) to the perforated interior wall 38 which has a frustroconical shape and a round, horizontal top. As illustrated, the exterior wall 36 has a frustroconical shape with this wall extending parallel to the interior wall 38. Alternatively, the exterior wall 36 can be constructed so as to have four sides with opposite sides converging in the upwards direction.

The cross-section of FIG. 4 shows the cross-section of the stack member about midway along its height. As illustrated, the stack member has a square, exterior horizontal cross-section formed by four sides of exterior wall 20 made of sheet metal. However, the perforated interior wall 22 has a circular horizontal cross-section. Extending inwardly from the interior wall 22 are four metal braces 86 which again can be welded or bolted to the interior wall and can be made of ¼ inch flat bar. The inner ends of these braces can be rigidly connected to a reinforcement ring 88 which provides support for an elongate, central, cylindrical tube 90. The tube 90 has a central longitudinal axis which is co-axial with the central axis A. The tube 90 forms a hollow passage through which passes the connecting member or rod 72.

FIG. 5 shows a horizontal cross-section similar to that of FIG. 4, with this cross-section being further down on the stack member. Shown in this view is a further set of four braces 92 which are connected at their outer ends to the interior wall 22. The inner ends of these braces are connected to a reinforcement ring 94 which also is used to support the tube 90. There can also be mounted on the exterior wall in the vicinity of the braces 92, horizontal, tubular frame members 96 which are connected to one another at the corners of the stack.

The illustrated embodiment of the gas stack also has the aforementioned elongate, central airflow defining member 100 which has a substantially cylindrical exterior shell 102 made of sheet metal perforated with numerous small holes distributed over at least a major portion of this shell. The member 100 can extend from a bottom end 104 located in the region of the inlet end 14 of the stack to a top end 106 located in the region of the top of stack member 12 and adjacent to the plug member. In order to reduce sound emitted from the stack, sound attenuating material 108 can be arranged within the exterior shell in the region where the numerous small holes of the shell are located. In order to protect the material 108 from moisture or rain entering the stack, a moisture proof plastic barrier layer (not shown) is arranged between the material 108 and the exterior shell 102. One suitable known plastic layer is sold under the trademark TEDLAR™ and another suitable material is sold under the trademark MYLAR™. These plastic layers still allow sound attenuation to take place. It will be understood that if quiet operation of the stack is not required, then the sound attenuating material, which can be fiberglass batting or mineral wool, can be omitted and the shell 102 entirely can be made of imperforate sheet metal. As illustrated, the top of the portion of member 100 containing the material 108 can be closed by horizontal metal plate 110 while the bottom of this portion can be closed by a round, horizontal plate 112. Each of these plates is fitted with a central hole for passage of the elongate rod 72. A bottom portion of the member 100 forms a cylindrical hollow chamber 114 which accommodates the actuator 62. Also, located in this chamber is an actuator support 116. An upper portion 118 of the member 100 is also hollow and is open at the top. The aforementioned skirt 66 of the plug member extends into this upper portion and is able to move upwardly or downwardly therein. The cylindrical exterior surface of the skirt 66 is preferably sized and arranged to be close to the interior surface of the upper portion of member 100 and, in the lowermost position of the plug member shown in FIG. 2, the skirt 66 can be immediately adjacent the interior surface of the shell which is slightly tapered in the downward direction.

It will be seen that the airflow defining member 100 has a central, longitudinal axis which is coaxial with the central, longitudinal axis A of the stack member. Furthermore, the exhaust gas passage 18 is an annular passage along the length of this airflow defining member. As shown in FIGS. 1 and 2, in an exemplary embodiment of the stack, the transverse width or the diameter D of the airflow defining member increases gradually in the axial direction towards the outlet end of the stack member. By restricting the effective width of the exhaust gas passage 18 in this manner, the velocity of the exhaust gases in the exhaust stack can be maintained and even increased, thereby assisting in maintaining the required exit velocity of the exhaust gases at the top end of the stack. Also, at the bottom end of the stack, the effective width W of the annular passage around the airflow defining member should be sufficiently large to receive the airflow coming from the fan outlet when a fan unit is mounted below the stack (such as shown in FIG. 7 and described below). However, it can be desirable to have the airflow defining member wider at its top end in order to accommodate the skirt 66 and a reasonably wide plug member. As indicated, the narrowing of the effective width of the annular passageway at the top end of the stack member causes the airflow velocity to increase at the top end which is desirable to expel the exhaust gases upwards into the atmosphere as much as possible.

FIG. 7 illustrates the possible use of twin exhaust gas stacks arranged one beside the other, these stacks being indicated by references 120 and 122. FIG. 7 also illustrates the manner in which an exhaust stack of the present disclosure can be mounted according to a typical installation and how an exhaust stack of this type can be mounted in conjunction with air handling and air treatment components. Shown in FIG. 7 is a portion of a roof slab 124 which is a portion of the roof of a building which has not been illustrated. This building can be an industrial building, factory or laboratory. Located in the building is an air handling housing 126 which can have access doors 128 as shown. There can be located in the housing 126 heat recovery coils and air filters (not shown) which can be of standard construction. Mounted on a suitably strong roof 130 of the housing are two vane-axial fan units 132 and 136. In a system employing only a single exhaust stack, there would normally only be a single fan unit. Although a vane-axial fan unit is illustrated, it is also possible for the fan unit to be a centrifugal fan in which case the fan outlet is generally rectangular and a transition section is provided then in the exhaust stack to change from a rectangular internal cross-section to a circular internal cross-section. In a known manner, each fan unit can be mounted by means of a coil spring system indicated at 138 which reduces vibration from the operation of the fan. Mounted on top of each fan unit (which is the outlet side) is a respective one of the exhaust gas stacks 120, 122.

Mounted on the outside of each exterior wall 20 of each stack is a mounting flange 140 which can be a four sided flange with a square periphery, this flange extending along the sides of its respective stack which can have a square, horizontal cross-section along at least a lower section extending from the bottom end of the stack member to the flange. Suitable stack supporting sleeves 142, 144 can be mounted on the roof 124 and each extends around an opening 146 in the roof. Each mounting flange can be connected by means of nuts and bolts to a connecting flange provided on the top end of each sleeve 142, 144. The portion of the stack member above the mounting flange 140 can either have a circular exterior cross-section or a square exterior cross-section. It is also possible for the exterior of the stack to have a circular cross-section throughout the entire length of the stack member 12.

FIG. 6 illustrates an optional feature that can be fitted on the present exhaust gas stack 10. In particular, there can be provided an air induction collar 150 extending around and mounted on the tapered end section 28. This collar, during use of the exhaust stack, can provide induction of outside air into the flow of exhaust gases from the second end 32 of the end section 28. By providing an induction collar in this manner, outside air, which enters through the bottom end 152 of the collar, effectively dilutes the exhaust gas concentration. As a result, the overall stack height can be reduced from the stack height that would otherwise be required to meet local standards. For example, in one embodiment of an exhaust gas stack with an induction collar mounted thereon, the overall height of the stack is reduced from thirty feet to twenty feet.

The use of an induction collar or windband is taught in U.S. Pat. No. 4,806,076 which issued Feb. 21, 1989 to 1. S. Andrews and the description and drawings of this prior patent are incorporated herein by reference. As indicated in this existing patent, the induction collar can be secured to the exterior of the tapered end section 28 adjacent its upper outlet and in spaced relation to its exterior surface by means of suitable connecting brackets (not shown) which do not interfere with the air flow through the collar. The illustrated induction collar is in the form of a frustoconical collar which tapers upwardly and which can be made of a suitable sheet metal. The collar has an open top at 154 which can be located well above the top end 32 of the end section 28. If desired, the collar can also have an annular interior wall 156 which can also be made of imperforate sheet metal. The interior wall can have a lower portion 158 that tapers upwardly from the bottom end of the induction collar to a narrow throat at 160. Above this throat, the interior wall diverges.

An additional optional feature of a stack constructed with a central airflow defining member 100 is the provision of a drain system 170 shown in FIGS. 1 and 2. This system is used to drain off any water that enters the hollow upper portion 118 of the member 100. An inlet 172 of this drain system is arranged above the substantial portion of the exterior shell of member 100 that contains sound attenuating material. The system 170 can include a one inch diameter pipe that extends through and projects from the wall of the stack.

The schematic illustration of the stack 10 shown in FIG. 8 illustrates an exemplary version of a control system for controlling operation of the linear actuator 62. This control system includes the aforementioned electronic controller 80 which can be connected to a booster relay 172. The booster relay is used to get faster response from the actuator 62 when a signal is sent from the controller. It amplifies the volume of the control air in the circuit. Both the relay and the controller are connected by respective air lines 174 and 176 to a source 178 of pressurized air. This source can provide pressurized air in the range of 20 to 30 psig. The two electrical lines 78 and 79 that connect the sensor 76 to the controller can extend through stainless steel tubes arranged on the interior of the stack. These steel tubes extend to two fittings 180. Extending between each fitting 180 and the controller can be relatively short flexible vinyl tubes through which the electrical lines extend. Both the controller 80 and the booster relay 172 can be preset in the factory. The controller and relay 172 can be mounted on the side of the stack or inside the building (and not necessarily on the stack). If they are mounted on the stack in a manner that would otherwise expose them to the elements, then a protective weather guard (not shown) is provided.

One embodiment of an exhaust gas stack constructed according to the present disclosure (without the optional induction collar) was constructed on the basis of the following design parameters for the flow of the exhaust gases:

Maximum Flow: 26,400 cfm (Emergency mode)

Maximum Flow: 20,000 cfm (Normal mode)

Minimum Flow: 9,000 cfm (Normal mode)

Average Flow: 17,500 cfm (Assumed)

In the case of a conventional exhaust gas stack operating at 20,000 cfm continuously, the required BHP of the axial vane fan is approximately 6.3 horsepower. On the other hand, with an exhaust gas stack according to the present disclosure which is able to operate at an average flow of 17,500 cfm continuously, the fan power required is only 11.1 horsepower. On the basis of this comparison and assuming twenty-four hours per day operation and electricity costs of ten cents per kW, the energy savings per year are approximately $3,600.00 per fan. It should be noted that this comparison between the power requirements for the fan when an exhaust gas stack according to the present disclosure is used as compared to when a conventional exhaust gas stack is used, is based on the assumed need to maintain 4,000 fpm velocity discharge at all operating levels. The energy efficiency of the disclosed exhaust gas stack is primarily due to its variable air volume (VAV) operation capability. The airflow volume handled by an exhaust fan used with this gas stack is reduced as the exhaust gas volume is reduced when compared to prior exhaust systems that introduce outside air in order to maintain flow velocity. As a result, the disclosed exhaust gas system requires less power than prior exhaust gas systems. The present exhaust gas system can be a VAV apparatus since the outlet velocity is maintained over at least a range of exhaust airflow rates by adjusting the opening area at the stack discharge.

While the present invention has been illustrated and described as embodied in certain exemplary embodiments, it is to be understood that the present invention is not limited to the details shown herein, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the disclosed exhaust gas stacks and velocity control apparatus for use with such stacks may be made by those skilled in the art without departing in any way from the spirit and scope of the present invention. For example, those with ordinary skill in the art will readily adapt the present disclosure for various other applications without departing from the spirit and scope of the present invention. 

1. An exhaust gas stack comprising: an elongate, tubular stack member having an inlet end, an outlet end, a central longitudinal first axis, and an exhaust gas passage extending between said inlet and outlet ends, said stack member adapted to extend vertically with said inlet end being a bottom end when said exhaust gas stack is installed for use; an end section mounted on said outlet end and having open first and second ends and a frustoconical passage extending between the first and second ends, said first end being connected to the outlet end of said stack member; an inner duct section, which is substantially circular in transverse cross-section, has a first duct end, an open second duct end, and a central, longitudinal axis coaxial with said first axis, said duct section being mounted centrally in said passage in the end section whereby a gas flow passage is formed around the duct section and between the duct section and the end section, said duct section having a first axial portion adjacent said first duct end and a second axial portion adjacent said second duct end with the two axial portions meeting at an annular junction of reduced diameter relative to the diameter of said first axial portion at said first duct end; an air plug member movable between an open position at which exhaust gases can flow past said plug member and through said inner duct section and another position at which said plug member restricts or prevents flow of exhaust gases through said inner duct section; and an actuator connected to said plug member and capable of moving said plug member between said open position and said closed position.
 2. An exhaust gas stack according to claim 1 wherein an annular duct wall forming said first axial portion of the inner duct section curves radially inwardly and upwardly as seen in cross-section through said central, longitudinal axis of the duct section and said second axial portion has a frustoconical shape which diverges upwardly from said annular junction.
 3. An exhaust gas stack according to claim 2 wherein said plug member has a hemispherical top portion with a convex exterior surface directed upwardly during use of said stack and a substantially cylindrical skirt extending from said top portion towards the inlet end of the stack member.
 4. An exhaust gas stack according to claim 3 wherein a section of said convex exterior surface of the plug member extends parallel to and is spaced apart from said annular duct wall when said plug member is in the open position.
 5. An exhaust gas stack according to claim 1 wherein an elongate central airflow-defining member is mounted in said stack member, extends along at least a substantial major portion of said longitudinal first axis, and has a top end arranged adjacent said air plug member.
 6. An exhaust gas stack according to claim 5 wherein said airflow defining member has a substantially cylindrical exterior shell made of sheet metal perforated with numerous small holes distributed over at least a substantial major portion of the exterior shell and containing sound attenuating material arranged within said exterior shell in the region of said numerous small holes.
 7. An exhaust gas stack according to claim 1 wherein said actuator includes a linear actuator unit mounted in said stack member along the longitudinal first axis and an elongate connecting member extending from said actuator unit to said plug member and operatively connected to said actuator unit so as to be axially movable thereby in order to move said plug member between said open position and said closed position.
 8. An exhaust gas stack according to claim 5 wherein said actuator includes a linear actuator unit mounted in said airflow-defining member, and said exhaust gas stack includes an electronic controller for controlling operation of said linear actuator and a sensor located at said end section for measuring velocity pressure of exhaust gases flowing from said end section and generating an electrical signal indicative thereof, said sensor being electrically connected to said controller and wherein said controller is programmed to operate said actuator unit from electrical input that includes said electrical signal so as to move said plug member to a controller calculated position amongst a range of positions between said open and closed positions in order to seek or maintain desired airflow velocity for the exhaust gases flowing from said end section of the stack.
 9. An exhaust stack according to claim 8 wherein said airflow defining member has a substantially cylindrical exterior shell made of sheet metal perforated with numerous small holes distributed over at least a substantial major portion of the exterior shell and containing sound attenuating material arranged within said exterior shell in the region of said numerous small holes.
 10. An exhaust stack according to claim 1 including an air induction collar extending around and mounted on said end section, said collar during use of said exhaust stack providing induction of outside air into the flow of exhaust gases from said second end of the end section.
 11. A velocity control apparatus for use at a top end of an exhaust stack, said apparatus comprising: a hollow member adapted for mounting on said top end of the exhaust stack and having open first end, an open second end of smaller diameter than the first end, and a frustoconical passage extending between the first and second ends, said first end being connected to said top end during use of said apparatus; an inner duct section having a first duct end, an open second duct end, and a central longitudinal axis which is aligned with a central longitudinal axis of said exhaust stack when said apparatus is mounted on the exhaust stack, said duct section being mounted centrally in said hollow member whereby a gas flow passage is formed around the duct section and between the duct section and the hollow member; an air plug member movable between an open position at which exhaust gases can flow past said plug member and through said inner duct section and another position at which said plug member restricts or prevents flow of exhaust gases through said inner duct section and thereby at least reduces the amount of exhaust gases flowing through said inner duct section during use of said apparatus; and an actuator connected to said plug member and adapted to move said plug member between said open and said another position.
 12. A velocity control apparatus according to claim 11 wherein said inner duct section has a first axial portion adjacent said first duct end and a second axial portion adjacent said second duct end, the two axial portions meet at an annular junction of reduced diameter relative to the diameter of said first axial portion at said first duct end, and said second axial portion has a frustoconical shape which diverges in an axial direction away from said junction.
 13. A velocity control apparatus according to claim 11 wherein said plug member has a hemispherical top portion with a convex exterior surface directed upwardly during use of said apparatus and a substantially cylindrical skirt extending from said top portion towards an inlet end of the exhaust stack during use of said apparatus.
 14. A velocity control apparatus according to claim 11 wherein said actuator includes a linear actuator unit adapted for mounting in said exhaust stack and an elongate connecting member capable of extending from said actuator unit to said plug member and of being connected to said actuator unit so as to be movable lengthwise thereby in order to move said plug member between said open position and said another position.
 15. A velocity control apparatus according to claim 11 wherein said actuator includes a linear actuator unit adapted for mounting in said exhaust stack and said apparatus further includes an electronic controller for controlling operation of said linear actuator and a sensor for measuring velocity pressure of exhaust gases flowing from said hollow member and generating electrical input signals indicative thereof, said sensor being electrically connected to said controller, and wherein said controller is programmed to operate said actuator unit upon receiving said electrical signals so as to move said plug member to a controller calculated position amongst a range of possible positions between said open position and said another position in order to seek or maintain a desired airflow velocity for the exhaust gases flowing from said hollow member.
 16. An exhaust gas stack comprising: an elongate, tubular stack member having an inlet end, an outlet end, and an exhaust gas passage extending between said inlet and outlet ends, said stack member adapted to extend vertically upwardly from said inlet end when said stack is installed for use, said stack member including an exterior wall, an interior wall made of perforated sheet metal having numerous small holes distributed over its surface and spaced inwardly from said exterior wall, and sound attenuating material sandwiched between said exterior wall and said interior wall; an elongate central airflow defining member mounted in said stack member and having a central, longitudinal axis which is coaxial with a central, longitudinal axis of said stack member, said exhaust gas passage being an annular passage along the length of the airflow defining member, said airflow defining member having a sheet metal exterior shell perforated with numerous small holes distributed over at least a substantial portion of the exterior shell and containing sound attenuating material arranged within said exterior shell; an end section mounted on said outlet end of the stack member and having an open first end, an open second end, and a frustoconical passage that tapers upwardly from the first end to the second end, said first end being connected to the outlet end of the stack member; and an exhaust gas velocity control mechanism having an inner duct section which is coaxial with and mounted in said end section and an air plug member movable between an open position at which exhaust gases can flow through said inner duct section and another position at which said plug member restricts or prevents passage of exhaust gases through said inner duct section, a further passage being formed around said inner duct section between this duct section and the end section.
 17. An exhaust gas stack according to claim 16 wherein said control mechanism includes a linear actuator connected by an elongate connecting member to said plug member and capable of moving said plug member between said open position and said another position, said linear actuator is mounted in a lower portion of said airflow defining member, and said connecting member extends along said longitudinal axis of the airflow defining member.
 18. An exhaust gas stack according to claim 16 wherein said air plug member has a rounded top portion adapted to engage an interior surface of said inner duct section in said another position and a substantially cylindrical skirt portion extending from said top portion towards said inlet end of the stack member and wherein said exterior shell of said airflow defining member is substantially cylindrical and has a substantially hollow upper portion, said skirt portion being mounted for telescoping, sliding movement in said hollow upper portion.
 19. An exhaust gas stack according to claim 16 including a drain system for draining off water that enters an upper portion of the airflow defining member via the second end of said end section, wherein an inlet of said drain system is arranged above the substantial portion of the exterior shell containing sound attenuating material. 